Ore fluid chemistry in super-giant porphyry copper deposits

Abstract

Laser ablation inductively Coupled Plasma Mass Spectrometry (ICPMS) analysis of fluid inclusions trapping mineralising solutions in major porphyry-copper deposits indicates that copper concentrations can reach 10_3 - 10_4 ppm in both dilute, CO_2 -bearing fluids and in high salinity brines. Non-C1 transport of Cu is implicated. Au has been determined at 0.2 - 0.4 ppm in the low salinity fluids and may be co-transported with Cu. In contrast, high concentrations of Fe, Mn, Zn and Pb are only attained in brines and correlate strongly with Cl-indicating chloride transport. Physical and/or temporal separation of these fluid types can therefore produce metal fractionation within porphyry systems. Evidence concerning the behaviour of Mo is less clear but brine transport appears to be favoured; phase separation is one mechanism that may account for the fractionation of Mo from Cu and the formation of quartz-molybdenite veins.